Apparatus for making prints from colour negatives

ABSTRACT

AN APPARATUS FOR MAKING PRINTS FROM COLOUR NEGATIVES HAS ADJUSTABLE FILTERS FOR CONTROLLING THE COLOUR COMPOSITION OF THE PRINTING LIGHT WHICH ILLUMINATES THE NEGATIVE BEING PRINTED. THE REQUIRED COLOUR COMPOSITION IS DEPENDENT ON THE COLOUR DENSITIES OF THE NEGATIVE. TO MEASSURE THE COLOR DENSITIES THE PRINTING LIGHT IS SAMPLED BOTH BEFORE AND AFTER TRANSMISSION THROUGH THE NEGATIVE AND THE SAMPLES ANALYZED IN RESPECT TO THEIR PRIMARY COLOUR COMPONENTS. THE CORRESPONDING COMPONENTS OF THE LIGHT SAMPLES BEFORE AND AFTER THE NEGATIVE ARE COMPARED TO   OBTAIN DIFFERENT SIGNALS REPRESENTING THE COLOUR DENSITIES OF THE NEGATIVE. THESE SIGNALS ARE USED IN TURN TO AUTOMATICALLY ADJUST THE FILTERS FOR THE CORRECT COLOUR COMPOSITION OF THE PRINTING LIGHT. A SHUTTER IS LOCATED BETWEEN THE NEGATIVE AND THE PRINTING MATERIAL AND IS OPENED TO EXPOSE THE LATTER ONCE THE PRINTING LIGHT HAS BEEN ADJUSTED. THE LIGHT SAMPLES ARE ALSO USED TO DERIVE BRIGHTNESS SIGNALS FOR CONTROLLING THE TIME FOR WHICH THE SHUTTER IS OPENED.

June 27, 1972 H. SCHAUB HAL 3,672,768

APPARATUS FOR MAKING PRINTS FROM COLOUR NEGATIVES Filed Aug. 24, 1970 3Sheets-Sheet 1 FIGJ June 27, 1972 H. SCHAUB ErAL 3,672,768

APPARATUS FOR MAKING PRINTS FROM COLOUR NEGATIVES Filed Aug. 24, 1970 3Sheets-Sheet 2 U08 18 0c; 16 U u q June 27, 1972 scH u ETAL 3,672,768

APPARATUS FOR MAKING PRINTS FROM COLOUR NEGATIVES Filed Aug. 24, 1970 3Sheets-Sheet 5 DOB 1 FIGA United States Patent Oflice 3,572,768 PatentedJune 27, 1972 3,672,768 APPARATUS FOR MAKING PRINTS FROM COLOUR NEGATIVES Heiner Schaub, Wettingen, Zurich, and Kurt Thaddey and Tino Celio,Buchs, Zurich, Switzerland, assignors to Ciba-Geigy AG, Basel,Switzerland Filed Aug. 24, 1970, Ser. No. 66,416 Claims priority,application Switzerland, Aug. 28, 1969, 13,065/ 69 Int. Cl. G03b 27/78US. Cl. 355-38 7 Claims ABSTRACT OF THE DISCLOSURE An apparatus formaking prints from colour negatives has adjustable filters forcontrolling the colour composition of the printing light whichilluminates the negative being printed. The required colour compositionis dependent on the colour densities. of the negative. To measure thecolour densities the printing light is sampled both before and aftertransmission through the negative and the samples analysed in respect oftheir primary colour components. The corresponding components of thelight samples before and after the negative are compared to obtaindifference signals representing the colour densities of the negative.These signals are used in turn to automatically adjust the filters forthe correct colour composition of the printing light. A shutter islocated bet-ween the negative and the printing material and is opened toexpose the latter once the printing light has been adjusted. The lightsamples are also used to derive brightness signals for controlling thetime for which the shutter is opened.

This invention relates to apparatus for making prints from colournegatives.

It is known in such apparatus to have filters arranged between a lightsource and the negative to be printed, by means of which filters theproportion of the primary colour components of the light of the lightsource can be controlled. Additionally light blending or mixing deviceis arranged in the light path between the filters and the negative tomix the primary colour components to produce a uniform printing light.To control the colour composition of the printing light for printingfrom a given negative, there is provided a colour intensity regulatingsystem selectively sampling the printing light by means of aphotoelectric device. The system so controls actuating means for thefilters that the light illuminating the negative has the requiredspectral compositions. The known apparatus included a shutter arrangedin the light path between the negative and a plane at which printingtook place. After the filters have been adjusted, the shutter is openedby a time switch device for the period of time required to expose theprinting material.

Heretofore, in apparatus of the kind outlined above the colour intensityregulating system is responsive only to light which is transmitted bythe negative. This has the disadvantage that the measurement of thecolour density of the negative also includes the influence of the lightsource and the filters. It cannot be ascertained Whether the measuredvalue pertaining to the negative derives solely from the negative orincludes the effects of changes in the light source or the filters.

The present invention arises from the realisation that theabove-mentioned disadvantage can be mitigated by measuring the colourcomposition of the printing light before and after the negative.Consequently the integral density of the negative can then be accuratelydetermined, irrespectively of fluctuations of the light source orfilters. The colour components are adjusted by the colour filters.

In its broadest form the invention provides apparatus for making printsfrom colour negatives, comprising a light path including a source oflight, a plane at which a negative to be printed is receivable, a planeat which printing material is receivable, means for imaging a receivednegative at the printing plane, a plurality of filters interposed insaid path between said source and said negative plane, said filtersbeing selectively actuable to control the proportions of the primarycolour components in the light received at said negative plane, andmeans disposed between said filters and said negative plane to mix saidcolour components to produce a printing light for illuminating anegative received at the negative plane; first and second sampling meansdisposed at opposite sides of said negative plane to sample the lighttransmitted to said negative plane for illuminating a negative and thelight transmitted from said negative after transmission through thenegative respectively; means responsive to the primary colour componentsof the sampled light to produce a first set of signals representing theintensities of the primary colour components sampled by said firstsampling means and a second set of signals representing the intensitiesof the primary colour components sampled by said second sampling means;a respective means for comparing each primary colour signal of the firstset with the corresponding primary colour signal of the second set toderive a signal representing the difference between the intensities ofthat primary colour at said opposite sides of said negative plane; arespective means coupled to actuate each of said filters; and respectivemeans coupling each comparison means to that one of the actuating meansfor the filter controlling the primary colour with which the comparisonmeans is associated whereby each filter is actuated in dependence uponsaid difference signal of the primary colour controlled by the filter.

In order that the invention may be better understood an embodimentthereof will now be described with reference to the accompanyingdrawings in which:

FIG. 1 shows a schematic representation of the whole apparatus;

FIG. 2 shows a section along line 1111 of FIG. 1; and

FIGS. 3 to 6 are diagrams to illustrate the operation of the apparatus.

The apparatus of FIG. 1 comprises an opto-mechanical portion and anelectrical portion. The opto-mechanical portion comprises a light pathat one end of which is a printing light source 1. The source is followedby a filter system having yellow magenta and cyanogen filters 2, 3 and 4respectively, means (not shown) for the positioning of a negative in aplane indicated at 5, an objective lens 6, a light shutter 7 and means(not shown) for the positioning of printing material in a planeindicated at 8. The printing light source 1, which preferably comprisesone or more halogen lamps, is located within a reflector 9, whichcomprises a number of cold light mirrors. A heat filter 10 is arrangedat the output end of the reflector 9 to ensure that as little infraredradiation as possible passes to the negative plane 5. The yellow magentaand cyanogen filters 2, 3 and 4 are preferably interference filters.Each can be swung into the path of rays leaving reflector 9 to aselected extent, the filters being movable by respective drive motors11, 12 and 13. Thus the colour composition of the light received at thenegative plane is determined by the relative degrees of insertion of thefilters into the optical path between the source and the negative plane.The colour composition is controlled by the filters above specified interms of the blue, green and red components. Additionally light blendingor mixing means are disposed in the light path between the filters andthe negative plane. After passing through the filters 2, 3 and 4 thelight passes into a first mirror tunnel 14 for the mixing of the blue,green and red components.

A pyramid 15 of material with low diffusion capacity ensures a furthermixing of the light. The light passes through the pyramid 15 into asecond mirror tunnel 16 which is closed on the output side by atranslucent ground glass screen 17. In the second mirror tunnel afurther mixing of the light takes place. The second mirror tunnel isprovided with corner mirrors 18, which reflect the light from thepyramid 15 in such a way that the corners of the ground glass screen 17are also adequately illuminated, By virtue of the arrangement andspecial design of the second mirror tunnel, uniform illumination of theground glass screen 17 and hence of the negative is accomplished. Thelight leaving the screen 17 may be conveniently referred to as theprinting light. When the shutter 7 is opened, the negative illuminatedby the printing light is projected through the objective 6 and imaged atthe printing plane 8.

In the path of the printing light, four fibre optic, light guides 19,20a, 20b, and 21 are arranged to sample the printing light before andafter the negative plane. The light guide 19 picks up the printing lightof intensity before the negative, whereas the light guides 20a, 20b, and21 pick up the printing light of intensity after the negative beingprinted. The three light guides a, 20b and 21 are combined together to acommon fibre optic guide 22. The light picked up by the light guides isseparated into the colour components blue, green and red by means of afilter wheel arranged between the light output ends of the light guides19 and 22 and a photo-detector 24. The filter wheel is rotatable aboutits axis by a motor 23. The manner in which the colour components areseparated will be described with reference to FIGS. 2 and 3.

As can be seen from FIG. 2, the filter wheel 25 comprises an opaque discin which there is provided a blue filter 26, a green filter 27, and ared filter 28 angularly (circumferentially) displaced but at the sameradial distance from the wheel axis. Position-defining indicia 30 arearranged in concentric tracks 29 at the periphery of the wheel andcooperate with a scanner 33. Furthermore a window 31 is arranged in thefilter wheel for periodically unmasking a reference lamp 32 arranged inits track. The window 31 is radially inwardly displaced from the colourfilters and is at a different circumferential (i.e. angular) position.The position indicia 30 provide information on the instantaneousposition of the filters relative to the position of the ends of thelight guides 19 and 22 on the upper side of the filter wheel as thelatter rotates. This information is detected by means of the scanner 33.The light guides 19 and 22 are disposed adjacent one another in thetrack of the colour filters so as to be successively swept by each ofthe filters in turn.

Upon rotation of the filter wheel 25 by the motor 23, the colour filtersblue 26, green 27 and red 28 are cyclically inserted into the path ofthe light between the ends of the light guides 19, 22 and the photocell24 on opposite sides of the filter disc. Consequently the output of thephotocell 24 comprises two sets of signals a first of which cyclicallyrepresents the blue, green and red separation components of the lightintensity 4: These primary colour component signals are interleaved withcorresponding components of the light intensity 3 which constitute thesecond set, the light sample being picked up integrally by the lightguides 20a, 20b. The light of the reference light source 32 also passesperiodically through the window 31 to the photo-cell 24 at a differenttime in the cycle of rotation of the filter wheel. With the help of thisintensity reference it is possible to compensate for drift in thesubsequent measuring circuitry as is disclosed hereinafter.

The pulse sequences appearing at the output of the photo-cell 24 as aresult of the arrangement described by way of example, is presented inthe diagram of FIG. 3 as a function of time. On each cycle of rotationof the filterwheel 25 the following seven electrical quantities appear 4at the output of the photo-cell in succession corresponding to thefollowing light intensities:

U corresponding the light intensity of the blue component of theprinting light before the negative;

U corresponding to the light intensity of the blue component 5 of theprinting light after the negative;

U corresponding to the light intensity of the green component before thenegative;

U corresponding to the light intensity of the green component 5 afterthenegative;

U corresponding to the light intensity of the red component before thenegative;

U corresponding to the light intensity of the red separation after thenegative; and

U corresponding to the intensity 11 of the reference lamp 32.

The outputs of the photo-cell 24 and the scanner 33 are the inputs ofthe electrical portion of the apparatus which will now be described. Theoutput signals U U13, UOG, UIG, UQR, U and U Of the photocell areanalogue values and are converted into logarithmic converter stage 34 toform the signals D n, D D D Don, D and D having logarithm-representingvalues. [It is convenient to convert the analogue values intologarithms. In this way several orders of intensity values can beprocessed with a constant relative error. Correction values are summatedin densities and not in transmissions. After conversion to logarithmicform, the seven resulting signals D D113, D D Don, D and D pass througha distributor or scanned switching device 35 and are then storedindividually in seven stores. The distributor is controlled by theposition information from scanner 33, and connects the photocell 24toits several outputs in succession in synchronism with the rotation ofthe wheel 25. Thus the signals of the first and second sets are passedonly into the respective stores assigned to them. For simplicity onlythe stores for the blue channel are shown in the block diagram. Thestore 37 is for the blue component (D before the negative being marked37, and store 38 is for the blue component (D after the negative. Thedistributor 35 has a further scanned output by which the measured valueof the intensity of the reference lamp 32 is set to a further store 36.In a comparator stage 42 this measured value is compared with a selectedreference value 53. If the measured value deviates from the referencevalue, this means a drift of the operating point of the stage 34. Thecomparator stage 42 then transmits an error signal to the stage 34:which compensates this drift in such sense as to reduce the errorsignal. In this way drift of both the photo-cell 24 and of the stage 34is compensated.

Before going further with the signal processing, it should be noted thatthe term summation stage will be used herein for stages effectivelyperforming addition or substraction or both, since the mathematicalfunction performed depends on the polartities of the signals applied toany such stage. These relative polarities are shown by the or signapp-lied to the various signal inputs of a summation stage.

The output of the D store 37 is led directly, and that of the D store 38by way of a summation stage 78, to a summation stage 44 from which thedifference value D is obtained. The output D of the summation stage 44is taken to the respective inputs of a slope correction stage 47, asummation stage 46 (performing a subtraction function) andaverage-calculating stage 73. The output of the stage 46 is connectedvia an undercorrection stage 45 (explained below under the operation ofthe apparatus) to one of the addition inputs of summation stage 63(performing both addition and subtraction operations). The output of theslope correction stage 47 also goes to an addition input of stage 63.The average calculating stage 73 is connected in the same way as shownfor the blue channel to the D D summation stages corresponding to stage44 of the red and green channels. A subtraction input of the stage 63 isconnected to the output of the D store 37. The stage 63 has furtheraddition inputs to signal-providing stages 58, 54 and 55, which provideinputs relating to manual corrections (58), negative parameters (54) andprinting material parameters (55) respectively.

The output of the summation stage 63 is applied to a blue channelservo-amplifier 50 which drives the actuating means for positioning theyellow filter 2, the actuating means being the servo-motor. A likearrangement is used for all three colour channels and is therefore shownin the drawing only for the blue channel (indexv B).

The outputs of the D D and D -stores, only the D -store 37 being shown,are lead to an averagecalculating stage 57 which averages the threeinputs. The output D of the average-calculating stage 73 is connected tothe input of a brightness-correction stage 61. The output of the averagecalculation stage 57 is conneoted to the input of summation stage 64(which performs addition). Stage 64 has further inputs from thebrightness correction stage 61 and the correction stages 58, 54 and 55for manual corrections (58), negative parameters (54) and printingmaterial parameters (55). The output of stage 64 is applied a timer 74controlling a release mechanism 74 for the shutter 7, the shutteropening being dependent on the value of signal from stage 64. The timer74 is connected further to the output of a blocking stage 71 havingthree inputs one of which is connected to the outputs of the summationstage 63 of the blue channel and the other two of which are connected tothe outputs of the corresponding summation stages (not shown) of the redand green channels. The blocking stage 71 releases the timer 74 foroperation only when the signal at each input of the blocking stage fallsbelow a certain value (threshold value).

The manner of operation of the apparatus so far described will now beconsidered in more detail with the help of the diagrams of FIGS. 4 to 6.

If a print which is correct in the colour reproduction is to be producedfrom a standard negative, this can be done only with certain positionsof the yellow, magenta and cyanogen filters 2, 3 and 4 respectivelygiving a particular colour composition of the printing light before thenegative.

The diagram of FIG. 4 is a plot of the values D against D wherein thelogarithm of the blue intensity before the negative is plotted asfunction of the density of the blue separation of the negative itself.The filter position of the yellow filter 2, which controls the value Dat correct printing of the standard negative, symbolized by index St, isindicated by point 100. Now, if a negative is inserted which is denserthan the standard negative but which is balanced in respect of colour,the slope correction stage 47 and the regulating circuitry will ensurethat the yellow filter swings out, so that the same blue intensity actsat the printing plane as in the standard negative. This correction isaccomplished in the example shown in FIG. 4 by swinging out of theyellow filter to the position symbolized by point 101. Usually it isappropriate and may be even necessary, to influence this relationship,which is dependent on the density of the negative, so as to compensatefor disturbing parameters, such as the Schwarzschild effect and othereffects deriving, for example, from the paper development. For thispurpose the straight line relationship 102 has its slope changed butstill intersects the standard point 100: i.e. line 102 is turned throughan angle about the point 100, so that it assumes a position such as isshown by line 103. This socalled slope-adjustment takes place in theslope-correction stage 47 and may be positive or negative or zero. Thusthe slope of the straight line 103 may be set greater or smaller thanand, of course also equal to, that of the straight line 102.

The difference signal fed by the summation stage 46 into theunder-correction stage 45 indicates whether the negative being printedhas a colour dominant or a colour cast, but it is independent of whetherthe negative is over or under exposed, since the brightness component Dhas been subtracted from the colour component D by stage 46.

By means of the under-correction stage 45 the effectiveness of the graycompensation can be adjusted in the sense of an under-correction.Without under-correction a negative is printed in colour so that on thepositive a print is made which has equal blue, green and red components.The so-called grey criterium is then fully eifective. Negatives with acolour cast are correctly printed in this manner. Now if in a pictureone colour component dominates (take as an example a picture of bluesky, lake and small white yacht) the regulating circuitry cannotascertain whether the negative has a cast or whether in the subject ofthe picture, in fact, the blue dominates. With complete greycompensation a small white area (eg the yacht) in large blue field (sky)would be printed with a yellow cast. This deficiency can be eliminatedby under-correction. An under-correction, eg in the blue, has the effectthat the blue dominant of the negative in the blue is only partlyentered as a parameter for the control of the yellow filter. At completeunder-correction in the blue, the blue component of the printing lightis equal to that of the standard negative and is, therefore, independentof the blue component of the particular negative inserted.

The slope correction and the under-correction can be adjustedindependently of one another by means of stages 47 and 45.

The sum of the colour density signals appearing on the addition inputsof stage 63 each forms a specified desired value, which is a function ofthe density of the negative, the slope and under-correction adjustmentand the further parameters adjustable in stages 54, 55 and 58. The sumon the addition inputs is in elfect compared with the D signal (asubtraction input). The servo-loop controlling the yellow filter 2 actsto reduce the output of stage 63 at least below the threshold valuereferred to above.

The operation described for the blue channel is the same for the greenand red channels.

The diagram of FIG. 5 shows, as an example, the desired values S S S forthe blue, green and red channel expressed in voltage values (mv.). Inthis example the desired value S of the red channel is maximum. Theregulating circuitry now commands the servo-amplifier of the red channel(not shown) to turn the rotor of the servomotor 13 in such a way thatthe cyanogen filter 4 controlling the red component of the printinglight is completely removed from the path of rays. The position of theyellow and magenta filters 2 and 3 respectively is adjusted until thedesired and the actual value for each colour channel agree with oneanother. Only then can the shutter 7 be released by the action ofblocking stage 71 on timer 74. The actual valueD for the blue channel iscontained in the store 37; moreover the actual values D and D for thetwo other colour channels are stored (in stores not shown in thedrawing).

The logarithmic signal D corresponding to the blue intensity after thenegative is stored in the store 38 as voltage value; similarly, thesignals D and D corresponding to the other two colour channels arestored in other stores (not shown). In the stage 57 the average value ofthe three signals D B, D and D is formed. After taking into accountvarious adjustable parameters in the stages 54, 55, 58, such asenlargement factor, size of negative, objective data, paper parameters,the stage 64 produces a signal which is converted from a logrithm intoan ordinary number again and converted in to a pulse of a certainduration for the control of the release mechanism 70 for the shutter 7.Fluctuations of the voltage supplied to the projection lamp 1, as wellas changes of intensity caused by inserted colour filters and thenegative present at planes in the light path of rays,

are virtually completely compensated by the measuring device that hasbeen described. The time control has the advantage that with a mediumlamp power an optimum exposure time is possible. A constant time controlwould have to be designated for densest negative, which, as experienceshows, requires approximately five times the lamp power; or, at equallamp powers, an exposure time which is five times as long. In case ofequipment malfunction seizure of movement of the colour filters 2, 3 and4), compensation is no longer possible, whereupon the exposure isautomatically blocked and wrong exposures are avoided.

Some other features of the apparatus of FIG. 1 will now be described.

Frequently it is desirable to make the exposure time also dependent onthe brightness of the negative. This is done in stage 61 alreadymentioned. In stage 73 the average value of the blue, green and redcomponents of the densities of the negative is formed. By adjustment ofthe so-called brightness slope the effect of this measured average valuecan be adjusted before application to stage 64.

The diagram of FIG. 6 shows the function of the stage 61 (brightnessslope) in which the logarithm of the exposure time t is plotted as afunction of mean negative density D Let the tested standard negative,which is again symbolised by index St, be correctly printed with anexposure time t at point 113. Without the effect of the brightness slopethe exposure times for negatives with deviating brightness would be onthe straight line 110. With the effect of the brightness slope, e.g.according to straight line 1.11, negatives, the means density D of whichis greater than the mean density D of the standard negative will beexposed for less time than would otherwise be the case, while brightnegatives will be exposed for longer than without brightness slopecorrection.

The two light guides 20a and 20b pick up the printing light integrallyfor what we shall call an integral measurement. The light guide 21serves for what we shall call a partial measurement in the printingplane and can be displaced with its input cross-section in that plane.Either the two light guides 20a and 2011 or the light guide 21 are usedin making a measurement, or are connected to common optical guide 22respectively. An auxiliary circuit, consisting of the summation stage78, a switch 79 and an auxiliary voltage source 80, ensures that thedifferences of intensity arising between the integral and the partialmeasuring method have no effect on the measured result. Three furtherswitches 77, 76 and 75 serve for the manual operation of the lightshutter 7 and for the blocking of the servomotor 11. The shutter 7 mustbe opened for a measurement to be made with the light guide 21, noprinting material being present at plane 8; the switch 75 is closedtogether with the corresponding switches in the other channels. Thecolour filters 2, 3 and 4 are then adjusted in their servo-loops asdescribed above. By manual opening of the switch 75 movement of thecolour filter 2 is blocked; similarly movement of the filters 3 and 4are blocked by opening the corresponding switches in the two othercolour channels. The shutter 7 is closed by the switch 76. Afterinsertion of the printing material the shutter 7 can be released bymeans of the switch 77 formed as a key controlling the timer 74 forexposure. To allow measurement and exposure to take place underidentical conditions, the voltage applied to the projection lamp 1 iskept constant by means of a stabilizer 81 inserted in the power supplyline.

Owing to the different measuring conditions of integral and partialmeasurement-which occur even with a negative uniform over its area andwith use of a constant light intensity -the light flux issuing from thelight guide 21 via the guide 22 will be different from that obtained byuse of the light guides 20a, 20b. In the store 38 different densitysignals will be stored depending on the method of measurement. In orderto receive identical density signals in both measuring methods, theselection switch 79 will be switched on at partial measurement to applythe auxiliary voltage U to the summation stage 78. The voltage U has itspolarity and absolute value so chosen that at the output signal of thestage 78 is independent of the measuring method. For dilferentenlargement factors and light guides naturally also several voltagesources 80 can be selectively switched onto the summation stage 78.

In summary it will be realised that the described apparatus providescompensation for the effects of colour changes in the printing lightsource or the control filters while allowing the latter to be adjustedin respect of the colour densities of each negative presented forprinting.

What is claimed is:

1. Apparatus for making prints from colour negatives, comprising a lightpath including a source of light, a plane at which a negative to beprinted is receivable, a plane at which printing material is receivable,means for imaging a received negative at the printing plane, a pluralityof filters interposed in said path between said source and said negativeplane, said filters being selectively actuable to control theproportions of the primary colour components in the light received atsaid negative plane, and means disposed between said filters and saidnegative plane to mix said colour components to produce a printing lightfor illuminating a negative received at the negative plane; first andsecond sampling means disposed at opposite sides of said negative planeto sample the light transmitted to said negative plane for illuminatinga negative and the light transmitted from said negative aftertransmission through the negative respectively; means responsive to theprimary colour components of the sampled light to produce a first set ofsignals representing the intensities of the primary colour componentssampled by said first sampling means and a second set of signalsrepresenting the intensities of the primary colour components sampled bysaid second sampling means; a respective means for comparing eachprimary colour signal of the first set with the corresponding primarycolour signal of the second set to derive a signal representing thedifference between the intensities of that primary colour at saidopposite sides of said negative plane; a respective means coupled toactuate each of said filters; and respective means coupling eachcomparison means to that one of the actuating means for the filtercontrolling the primary colour with which the comparison means isassociated whereby each filter is actuated in dependence upon saiddifference signal of the primary colour controlled by the filter.

2. Apparatus as claimed in claim 1 further comprising means connected toall said comparison means and responsive to said colour componentdiiference signals to provide a signal representing the average thereof;and wherein each coupling means comprises a stage having inputs coupledto the output of the associated comparison means and to the output ofsaid averaging means to combine the output signals therefrom, and anunder-correction stage responsive to said combined signals to modify thevalue thereof, the output of said under-correction stage being coupledas a signal independent of the density of a received negative to controlthe associated filter actuating means.

3. Apparatus as claimed in claim 2 wherein said light path furthercomprises a shutter disposed between said negative plane and saidprinting plane to control the transmission of light to said printingplane; and further comprising means including a timer to open saidshutter to allow light to impinge on printing material received at saidprinting plane for a desired period of time, said timer having an inputfor reeciving a signal the value of which determines said period oftime, means responsive to said first set of signals to provide a signalrepresenting the average value of the signals of said first set, andmeans coupled to said averaging means to combine said first set, averagevalue signal with said signal representing the average of said colourcomponent difference signals to produce a resultant signal, said timerinput being coupled to be responsive to said resultant signal.

4. Apparatus according to claim 1 wherein said means responsive to theprimary colour components of the sampled light includes a logarithmicconverter for producing said first and second sets of signals inlogarithmic form.

5. Apparatus as claimed in claim 1 wherein said sampled light-responsivemeans comprises a rotatably mounted opaque body having a set of filtersfor said primary colour components disposed at different angularpositions therein, said body bearing angular position-indicativeindicia, means arranged to detect said indicia as the body rotates, aphoto cell arranged to one side of said body to respond to lighttransmitted by said filters therein; and switching means connected tosaid photoelectric device and having a plurality of outputs to which thephotocell is successively connectable, said switching means beingcontrollable by said position-indicative indicia detector to scan saidoutputs in synchronism with the rotation of said body; and wherein saidfirst and second light sampling means terminate at the other side of thebody to direct the sampled light toward said photoelectric device, thefilters in the body being disposed to successively transmit light fromeach light sampling means to said photoelectric device upon rotation ofthe body whereby separation of the sampled light into primary colourcomponents at the photoelectric device is effected to produce said firstand second sets of primary colour components signals in a sequencetherefrom, said switching means operating to di- 10 rect each primarycolour component signal to a respective output thereof.

6. Apparatus as claimed in claim 5, wherein said sampledlight-responsive means includes a logarithmic converter connectedbetween said photo-electric device and said switching means to convertsaid primary colour component signals to logarithmic form.

7. Apparatus as claimed in claim 6 further comprising a window in saidfilter body, a reference light source at said other side of the body toilluminate said photoelectric device through said window at apredetermined angular position of the body, said switching meansincluding a further scanned output at which is obtained a signalrepresenting the illumination of said photoelectric device by saidreference light source; means for comparing said reference light sourcesignal with a predetermined signal to produce an error signal, and meansin said logarithmic converter responsive to said error signal to controlthe conversion operation effected thereby so as to reduce said errorsignal.

References Cited UNITED STATES PATENTS 2,616,331 11/1952 Pavelle 35536 X3,554,642 1/1971 Zahn 35532X SAMUEL S. MATTHEWS, Primary Examiner R. A.WINTERCORN, Assistant Examiner US. Cl. X.R.

